1. Field of the Invention
The present invention relates to a piezoelectric sound generator which is applied to, e.g., a piezoelectric buzzer or a piezoelectric loudspeaker, and more particularly, it relates to a piezoelectric sound generator including a monolithic sintered body which is obtained by laminating a plurality of ceramic green sheets and electrodes and cofiring the same.
2. Description of the Prior Art
FIG. 2 is a schematic sectional view showing a conventional piezoelectric buzzer as an example of a conventional piezoelectric sound generator. Referring to FIG. 2, a laminated type vibrator 2 is adhered to a metal plate 1. The vibrator 2 is formed by laminating three piezoelectric ceramic plates 2a, 2b and 2c so as to reduce its impedance and increase its sound pressure.
In the piezoelectric buzzer as shown in FIG. 2, the piezoelectric ceramic plates 2a to 2c are previously fired separately and polarized in directions indicated by arrows in FIG. 2, to be integrally formed with electrodes 3a to 3d on the metal plate 1. The electrodes 3a and 3c are electrically interconnected with each other by an electric connecting part 4a formed on the outer peripheral portion while the electrodes 3b and 3d are electrically interconnected with each other by an electric connecting part 4b also formed on the outer peripheral portion. In the piezoelectric buzzer as shown in FIG. 2, the vibrator 2 is formed by the three ceramic plates 2a to 2c, and is capable of generating high sound pressure because of its reduce impedance.
Another prior device, disclosed in, Japanese Patent Application No. 226577/1984 in the name of the assignee of the present application, is a piezoelectric buzzer which is in the background of the present invention, although this related art has not yet been published. FIG. 3 shows the piezoelectric buzzer as disclosed in the Japanese Patent Application No. 226577/1984. Referring to FIG. 3, a monolithic ceramic vibrator 12 is adhered on a vibration plate 11 which comprises a metal or plastic plate. The ceramic vibrator 12 has three ceramic layers 12a to 12c, which are obtained by laminating three ceramic green sheets, alternating with layers of electrode paste adapted to form inner electrodes 13b and 13c, and cofiring the same. Electrodes 13a and 13d are formed simultaneously with the inner electrodes 13b and 13c or separately after the firing. The ceramic layers 12a to 12c are polarized in directions indicated by arrows in FIG. 3. The electrodes 13a and 13c are interconnected with each other by an electrode connecting part 14a formed on the outer periphery of the monolithic ceramic vibrator 12 while the electrodes 13b and 13d are interconnected with each other by an electrode connecting part 14b formed on the outer periphery of the monolithic ceramic vibrator 12. Thus, in the piezoelectric buzzer as shown in FIG. 3, the monolithic ceramic vibrator 12 is integrally formed, whereby the respective ceramic layers 12a to 12c can be made very thin. Hence the impedance of the vibrator 12 is reduced in comparison with that of the piezoelectric buzzer as shown in FIG. 2, and remarkably higher sound pressure can be obtained.
FIG. 4 is a side elevational view schematically showing vibration states of the conventional piezoelectric buzzer as shown in FIGS. 2 or 3. In the conventional piezoelectric buzzer, a ceramic vibrator 22 adhered to a vibration plate 21 vibrates upon application of a voltage so as to alternate between two bent states as shown in broken lines A and B in FIG. 4, thereby generating sound waves. The modes X of such vibration are inside the outer periphery of the ceramic vibrator 22 as shown in FIG. 4, whereby the outer peripheral portions of the ceramic vibrator 22 are displaced a considerable distance by the vibration.
On the other hand, the electric connecting parts 4a, 4b, 14a and 14b of the conventional laminated type piezoelectric buzzers are formed on the outer peripheral portions of the ceramic vibrators. Thus, the electric connecting parts 4a, 4b tend to 14a and 14b suppress the vibration of the ceramic vibrators, and as a result, it has not yet been possible to obtain the desired sound pressure at certain desired resonance frequencies. Such problems are not restricted to the piezoelectric buzzers as shown in FIGS. 2 and 3 employing the so-called laminated or unimorph type vibrators, but also exist in a piezoelectric buzzer employing a bimorph type and other related types of vibrator.